pm_ls430

Hello all, Great forum. I am picking up a 2004 LS430 on Thursday. I am getting a pretty good deal on it. It is the car I wanted, but it does not have navigation in it. It has the full digital screen for radio and climate, but navigation is not included. Does anyone know if it is possible to add a navi system that will integrate directly with the existing setup? To use the system as if it were factory installed? If so, any idea on cost? Would I need to go to a dealer? Thanks for your help!